Polydopamine-Derived Carbon: What a Critical Role for Lithium Storage?

Polydopamine-derived carbon materials have attracted tremendous attention owing to nitrogen heteroatom doping. However, it is challenging to estimate the effect of the morphology and porosity of the polydopamine-derived carbon for lithium storage. Here, we designed three polydopamine-derived carbon materials with different morphologies and porosity: carbon nanospheres (CNs), mesoporous carbon nanospheres (MCNs), and bowl-like mesoporous carbon nanoparticles (BMCNs) to evaluate the electrochemical performance. Such a bowl-like mesoporous structure combines multiple advantageous features, including mesoporous channels and shorter transmission distance, thus delivering a high specific capacity. In addition, our work provides new insight into the electrochemical performance of polydopamine-derived carbon and a useful reference for its future application in high-performance lithium ion battery (LIB) electrode materials.


INTRODUCTION
With increasing aggravation of energy crisis and environment pollution, the development of an environmentally friendly new energy to replace traditional fossil energy is essential for building a sustainable energy structure and a friendly eco-environment. Lithium ion batteries (LIBs) are the most competitive candidates of energy storage systems (ESS) and have a wide application in portable electronic devices and electric vehicles (Tarascon and Armand, 2001;Jiang et al., 2015;Liu et al., 2015;Li et al., 2019;Zhu et al., 2019). However, graphitic carbon, as currently the standard commercial anode material for LIBs, possesses low theoretical capacity (372 mA h g −1 ) and undesirable rate performance, which limit its application in a high-energy system (Dai et al., 2019;Zheng et al., 2019;Rao et al., 2020). Nitrogen-doped carbon nanomaterials have been widely explored as alternatives to commercial graphitic materials, showing high reversible capacity, high-rate performance, and high cyclic capability. A variety of nitrogen-doped carbon materials have been obtained by using urea Sun et al., 2019), polypyrrole (Qie et al., 2012;Liu et al., 2016), polydopamine (Jian et al., 2019;Qian et al., 2020), and biomass sources (Lang et al., 2019;Shen et al., 2019;Zhao et al., 2019) as precursors. Among these materials, polydopamine-derived carbon materials have been widely applied in the anode/cathode field due to that it can easily coat on virtually any surface by self-polymerization of dopamine in aqueous media. For instance, the Si@void@C "yolk-shell" structure was fabricated via a sacrificial template process, which delivered a high capacity of ∼1500 mA h g −1 at a current density of 1 A g −1 (Liu et al., 2012). Wang and coworkers designed interconnected N/O co-doped porous rod-like nanocapsules (N/O-CNCs) through a sacrificial template process, and a capacity of 310 mA h g −1 was obtained at 30 A g −1 after 11000 cycles . Recently, Mai and coworkers reported sandwichstructured mesoporous polydopamine (mPDA)/reduced graphene oxide (rGO) nanosheets and found that this composite electrode could display a high specific capacity (Wang N. et al., 2019). In addition, compared with the traditional hollow structure, the bowl-like morphology has received extensive attention due to its unique structural advantages (Liang et al., 2015;Qian et al., 2020). However, to the best of our knowledge, the influence of the morphology and porosity of the polydopamine-derived carbon for lithium storage have not been thoroughly evaluated as anode materials.
Herein, we have designed three polydopamine-derived carbon materials with different morphologies and porosities: carbon nanospheres (CNs), mesoporous carbon nanospheres (MCNs), and bowl-like mesoporous carbon nanoparticles (BMCNs). Importantly, the effect of the morphology and porosity of the polydopamine-derived carbon for lithium storage were evaluated in detail. We find that the mesoporous structure can shorten the diffusion path of Li ions and electrons in the electrode and accelerate the diffusion rate of the electrolyte. In addition, the bowl-like morphology has a shorter transmission distance than nanospheres, which can shorten the lithium ion and electron transmission paths. Therefore, BMCNs combine the advantages of both mesoporous channels and shorter transmission distance so that it can effectively relieve the stress contributing to superior long-term cycling stability. The obtained BMCN electrode delivers a large reversible capacity of 300 mA h g −1 after 1000 cycles, revealing outstanding long-term cycle stability at 2 C.

RESULTS AND DISCUSSION
The overall fabrication process for CNs, MCNs, and BMCNs is schematically illustrated in Figure 1a. First, CNs were synthesized by the polymerization of dopamine hydrochloride (DA) under the condition of basic aerobic conditions using ammonia as a catalyst. Second, the low-block copolymer F127 is used as the template, DA as the carbon source. TMB acts as the medium in the ethanol/water system, which interacts with the hydrophobic PPO segment of F127 and DA molecules, respectively. This promotes the formation of F127/TMB/DA composite micelles. Continuous cooperative assembly promotes the formation of the initially formed composite micelles along the particle interior to form mesopores. The polymerization reaction under oxygen conditions continues to grow, and finally MCNs were formed by carbonization treatment in an N 2 atmosphere. Third, TMB was ultrasonically transformed into an emulsion to form an "oilin-water" system. A block copolymer F127/TMB/DA composite micelle is formed at the TMB/water interface; this seed-mediated anisotropic growth process yields BMCNs. The color changes of the synthesis process are shown in Supplementary Figure S1.
The  Figure S4) show that BMCNs clearly display a bowl-like structure with an even particle diameter of ∼250 nm, and uniform mesopores can be distinctly observed.
X-ray diffraction (XRD) patterns (Supplementary Figure S5) of the three samples suggest that two broad peaks at about 23.5 • and 43.7 • were assigned to partly graphitized (002) and quasi-amorphous (100) structure diffraction modes (Jian et al., 2019;Nan et al., 2019). The Raman spectra of CNs, MCNs, and BMCNs are shown in Supplementary Figure S6. It is known that the two characteristic peaks located at ∼1350 and ∼1590 cm −1 correspond to disordered sp 3 carbon (D band) and graphitic sp 2 carbon (G band), respectively (Gayathri et al., 2019;Hu et al., 2019). In addition, the intensity ratios of D and G bands (I D /I G ) for CNs, MCNs, and BMCNs are 1.03, 1.04, and 1.05, respectively, indicating a relatively superior graphitic degree.
The chemical composition and electronic structure of the three samples were analyzed by X-ray photoelectron spectroscopy (XPS). Figure 2A presents three typical peaks of C1s, N1s, and O1s without any impurities in three samples. The spectrum of C 1s exhibits that the primary characteristic peaks at 284.6, 285.9, and 289.1 eV denote the C-C, C-N, and O-C=O of CNs, MCNs, and BMCNs ( Figure 2B), respectively (Zheng et al., 2014;Chu et al., 2018;Shi et al., 2020). Four obvious peaks of N 1s at 398.4, 399.7, 400.6, and 402.4 eV are related to pyridinic N, pyrrolic N, graphitic N, and oxidized N in Figure 2C (Chen et al., 2016;Liu et al., 2017;Peng et al., 2019). The doping of nitrogen is conducive to enhancement of the electrochemical activity and conductivity of the material (Ai et al., 2013;Mao et al., 2018;Wang et al., 2018). Moreover, the atomic ratio in Figure 2D presents that the proportions of the three elements C, N, and O of CNs, MCNs, and BMCNs are basically the same. The above results indicate that the phase composition of CNs, MCNs, and BMCNs is identical, and there are only differences in pore size and morphology. The specific surface area and porosity distribution of MCNs and BMCNs were obtained from the N 2 adsorption/desorption analysis. Figure 2E shows that both MCNs and BMCNs display representative type-IV isotherms with an evident hysteresis loop, indicating the presence of mesoporous structures (Hong et al., 2019;Tian et al., 2019;Yin et al., 2019). The Brunauer-Emmett-Teller specific surface area of BMCNs was 44.2 m 2 g −1 , which was larger than that of MCNs (19.9 m 2 g −1 ). As shown in Figure 2F, the pore size distribution curves calculated using the Barrett-Joyner-Halenda model confirmed that the pore diameters of BMCNs are mainly distributed around 3.1 nm, which are larger than MCNs (2.2 nm). The results show that BMCNs have richer mesoporous channels, which will facilitate the contact between the active material and the electrolyte and accelerate Li + and electron transport.
We investigated and compared the electrochemical properties of CNs, MCNs, and BMCNs as candidate anode materials for LIBs. The Li-ion reaction behavior of the three electrodes was tested via cyclic voltammograms (CVs), as exhibited in Figure 3A and Supplementary Figure S7. The 0.9-V peaks can correspond to the decomposition of the electrolyte followed by the formation of a solid electrolyte interface (SEI) layer (Cui et al., 2019;Xu et al., 2019;Boyjoo et al., 2020). The 0.01-V cathodic peak is ascribed to the intercalation and deintercalation of Li ions into graphene-like planes Ould Ely et al., 2019;Rao et al., 2020). The galvanostatic discharge/charge curves of the BMCNs at 0.1 A g −1 are presented in Figure 3B. The BMCN electrode delivers high first-cycle discharge and charge capacities of 868 and 507 mA h g −1 , respectively, suggesting an initial Coulombic efficiency (ICE) of 58.4%. The initial Coulombic efficiencies of CN and MCN samples are 61.4% and 55.3%, respectively. The irreversible capacity loss may be originated from the formation of an SEI layer during the first cycle and electrolyte decomposition Zou et al., 2018;Liu et al., 2020). Figure 3C shows the cycling performance of CNs, MCNs, and BMCNs at 0.1 A g −1 . After 100 cycles, the BMCN electrode delivers a significant reversible capacity up to 371 mA h g −1 . In stark contrast, CN and MCN electrodes can only deliver 277 and 171 mA h g −1 after 100 cycles, respectively. Discharge capacity and Coulombic efficiency of the first five cycles of CN, MCN, and BMCN electrodes are presented in Figure 3D. The BMCN electrode performs the best and delivers high discharge capacities. In addition, the Coulombic efficiency of the BMCN electrode is maintained at above 96% from the second cycle, which are superior to CNs and MCNs. The rate performance of CNs, MCNs, and BMCNs at various current densities ranging from 0.1 to 5 A g −1 is presented in Supplementary Figure S8. The BMCN electrode delivers reversible charge capacities of 448, 381, 339, 285, 243, and 184 mA h g −1 at current densities of 0.1, 0.2, 0.5, 1, 2, and 5 A g −1 , respectively, which are superior to CNs and MCNs. When the current density returns from 5 to 0.1 A g −1 , a high charge capacity of 425 mA h g −1 can be recovered. To better demonstrate the good lithium ion storage performance of BMCNs at high current densities, the long cycling performance was further investigated at 2 A g −1 (Figure 3E), the BMCN electrode which delivered a reversible capacity of FIGURE 4 | (A) Electrochemical impedance spectra and equivalent circuit (inset) of the CN, MCN, and BMCN electrodes obtained after 50 cycles at 2 A g −1 .
(B) Schematic illustration for the lithium-ion and electron transport in the CN, MCN, and BMCN electrodes. 300 mA h g −1 is retained, and the Coulombic efficiency closes to 100% after 1000 cycles, indicating outstanding cycling stability over a long cycle life.
The electrochemical impedance spectroscopy (EIS) measurements of three samples were conducted after the 50th cycles. All Nyquist plots are composed of a semicircle at the medium frequencies and an inclined line in the low-frequency region. The equivalent circuit is fitted using the ZSimpWin software. In general, the semicircle at the medium frequencies can be assigned to the interfacial transfer impedance of electrons (R CT ), and the slope spike denotes the diffusion ability of Li ions in the electrodes. R e represents the electrolyte resistance. The CPE (constant phase element) denotes the capacitance of the SEI films and the double-layer capacitance, and Z W is in relation with the Warburg impedance (Hao et al., 2018;Wu et al., 2019;Zhang et al., 2019). It can be observed that the semicircle of the BMCN electrode ( Figure 4A) is smaller than other samples; this proves that the ion transport within the BMCN electrode is further ameliorated. The outstanding electrochemical performance of the BMCN electrode could be explained by the bowl-like mesoporous nanostructure ( Figure 4B). Lithium ions can be stored reversibly between graphene-like planes. Comparing the electrochemical performance test results of CNs and MCNs indicates that the existence of mesoporous channels can provide convenient channels for lithium ions and electrons and accelerate the penetration of electrolytes, which can improve the electrochemical performance of electrodes. Differences in electrochemical performance between MCNs and BMCNs are mainly due to the unique advantages of the bowl-like morphology. In terms of geometry, this special bowl-like morphology has a shorter transmission distance than nanospheres, which can shorten the lithium ion and electron transmission paths.
In a nutshell, we successfully synthesized polydopaminederived carbon materials with different porosities or morphologies using three different methods. BMCNs were synthesized by an emulsion-induced self-assembly strategy, and MCNs were prepared according to a general nano-emulsion assembly method. CNs were synthesized by self-polymerization. Importantly, the effects of porosity and morphology on their electrochemical performance under the same conditions were studied in detail. Electrochemical test results show that the mesoporous structure can shorten the diffusion path of Li + and electrons in the electrode and accelerate the diffusion rate of the electrolyte. In addition, the special bowl-like morphology has a shorter transmission distance than nanospheres, which can shorten the lithium ion and electron transmission paths.

DATA AVAILABILITY STATEMENT
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.